It is already known that, as disclosed in U.S. Pat. No. 4,367,924, the electrical optical element employing the ferroelectric liquid crystal exhibiting the chiral smectic C-phase and the chiral smectic H-phase responds to the applied voltage at high speed, and have the memory property. It is expected to use ferroelectric liquid crystals to a large sized display having a number of pixels (picture elements), a high speed electronic shutter, and a polarizer and so on.
As typical driving method of the ferroelectric liquid crystal element up to now is that, as disclosed in U.S. Pat. No. 4,508,429, the voltage pulse which determines the optical transmitting state of the liquid crystal element is applied at the predetermined period to render the average of the voltage applied during the predetermined period equal to zero, thereby preventing the deterioration of the ferroelectric liquid crystal. However, the practical driving method which is disclosed in U.S. Pat. No. 4,508,429 is the static driving method, and the multiplex driving method which is the most useful driving method for the large sized display is not disclosed at all. It is already known that, the threshold voltage of the ferroelectric liquid crystal is varied in accordance with the pulse width of the applied voltage pulse, namely, the ferroelectric liquid crystal has "the dependence on the pulse width". However, the driving method disclosed in U.S. Pat. No. 4,508,429 have no consideration with respect to the dependence on the pulse width, and further the driving method is too difficult to realize in practice.
Accordingly, the presnt inventors provide the practical multiplex driving method in consideration of the dependence on the pulse width in Japan application No. 119,680 (see: Japan Laid-open 263124/85) and Japan application No. 177,818 (see: Japan Laid-open 55630/86). However, in the driving methods according to Japan application Nos. 119,680 and 177,818, since the average of the voltage applied to the ferroelectric liquid crystal is not considered in particular, there would be possible that the average of the voltage applied to the ferroelectric liquid crystal inclines to either of the positive or the negative, and thereby promoting the deterioration of the ferroelectric liquid crystal. Namely, the driving methods according to Japan application Nos. 119,680 and 177,818 have the disadvantage that the life of the liquid crystal element becomes short.